Compression is used to manage (often by reducing) the ‘dynamic range’ of a signal, which is the span between the softest and loudest parts: Used well, it can make your tracks sound more polished by controlling maximum levels and maintaining higher average loudness.
But this is also one of the most misunderstood topics in audio production, and it can be hard for beginners to get started...
The 4 'Key' parameters
There are 4 ‘key’ parameters to consider when dealing with audio compression: Threshold, Ratio, Attack and Release.
Even though other settings matter to get more control over your sound, having a good understanding of these first is crucial: These are the settings you should firstly try to find and figure out on a new plugin or gear you may want to start using.
Threshold is the first key parameter you must understand to use a compressor well:
It refers to how loud you want the signal to be before compression applies.
In other words, the compressor will activate only when the signal is louder than the threshold.
In this example, it is set to -18.7dB, meaning every time a sound is louder than this, the compressor will turn on.
The ratio determines how much compression is applied to a signal that goes over the threshold: Every signal that exceeds it gets compressed at this ratio.
A ratio of 2:1 indicates that a signal exceeding the threshold by 8 dB will be attenuated half down to 4 dB: This is called the ‘Gain Reduction’ (GR).
A 3:1 ratio can be considered moderate compression, 5:1 would be medium compression, 8:1 starts getting into strong compression and 20:1 and more would be considered as “limiting”.
‘Attack’ refers to the time it takes for the signal to become fully compressed after exceeding the threshold level.
Faster attack times are usually between 20 and 800 us (microseconds), while slower times generally range from 10 to 100 ms (milliseconds) or more.
Fast attack times may create distortion by modifying inherently slow-moving low frequency waveforms. Slow attack times prevent the compressor from being triggered too fast so the transient remains uncompressed.
Release is the time it takes for the signal to go from the compressed state back to the original non-compressed signal.
Release times will be considerably longer than attack times, generally ranging from a few ms to 2-5 seconds, depending on the compressor.
‘Classic’ compression operation will be to set the release time to be as short as possible without producing a “pumping” effect.
If the release time is set too short and the compressor is cycling between active and non-active, the dominant signal will also modulate the noise floor, resulting in a distinct “breathing” effect.